This invention relates to a molding sand adapted to be cured by a microwave radiation.
One of the conventional methods for making molds comprises forming a metal pattern, heating the metal pattern to an elevated temperature, applying a molding material including a thermosetting resin therein to the heated metal pattern thereby hardening or curing the molding material by heat conduction from the heated metal pattern and finally separating the metal pattern from the cured molding material. In this method, however, since the pattern is made of metal, the cost for making the pattern will necessarily become expensive and it is difficult to produce a complicated shape of pattern. Besides, the molding material is cured by the heat conduction from the metal pattern with producing a great deal of energy loss and therefore a poor energy efficiency is obtained.
Another conventional method for making molds includes forming a metal pattern, heating the metal pattern to an elevated temperature about 250.degree. C., coating a powdered facing agent on the heated metal pattern by spraying, applying a molding material including a thermosetting resin therein to the facing agent coated and still heated metal pattern thereby curing the molding material by heat conduction from the heated metal pattern and finally separating the metal pattern from the cured molding material.
Casting surface of cast product produced by using this mold is very fine and it is possible to obtain roughness of 200 RMS for aluminum and 300 RMS for FC material of Japanese Industrial Standard. This method too, however, has same problems associated with the metal pattern described above.
In order to solve problems associated with the metal pattern, two of the applicants of the present invention have developed a new method for making molds and filed a U.S. patent application Ser. No. 237,767 on Feb. 24, 1981. The application has been assigned to the same assignee of the present application. In that application there is proposed a method for making a mold, comprising the steps of: forming a pattern with a material which allows microwave to pass therethrough, covering the pattern with a molding material including a thermosetting resin and a dielectric material to the microwave, exposing the molding material covered pattern to a microwave radiation for a time sufficient to cure the molding material by heat generated therein, and separating the pattern from the cured molding material.
According to the above method, since the microwave radiation is employed for heating and curing the molding material, heat loss can be minimized and energy efficiency is significantly improved compared with a typical conventional method employing the heat conduction from a heated metal pattern for heating and curing the molding material. Molding material or sand disclosed in that application is a mixture of silica, phenol resin and graphite powders. When this molding sand is used in the microwave curing molding process, time required to cure the molding sand is extended to about 6 minutes and a mold thus produced has a relatively low transverse strength (about 25 kg/cm.sup.2).